Pressurised gas storage vessels are very well known and are often called “gas cylinders” even if not of cylindrical shape. The pressurised gas storage vessel can be used to store a permanent gas, that is a gas which cannot be liquefied by the application of pressure alone, or a non-permanent gas, that is a gas which can be liquefied by the application of pressure alone. Examples of non-permanent gases are carbon dioxide and nitrous oxide.
Particular problems, which will be outlined below, can arise when supplying a non-permanent gas from a pressurised gas storage vessel containing the non-permanent gas in liquid state under the pressure of a gaseous phase. Because the molecules of the non-permanent gas will be contained in the liquid phase, it is necessary, if it is desired to supply the substance in the gaseous phase, to cause the liquid phase to vaporise. Such vaporisation occurs naturally while gas is withdrawn from the pressure vessel. The boiling liquid absorbs heat from the walls of the pressure vessel. The vessel has a given thermal mass and with large gas cylinders, say having a water capacity from 5 liters to 10 liters, and normal demand rates, this thermal mass and its surface area are sufficient to meet the demand for the gas. With vessels of a much smaller capacity, say, less than 1 liter, there is a lower thermal mass than surface area on which the boiling liquid can draw. In practice, depending on the rate of gas removal, small cylinders can reach low temperatures such as −30 to −700 C. As a result, the cylinders become potentially hazardous as they would create a cold burn if they came into all but the most fleeting contact with human skin. Further, low temperatures can cause problems downstream, through condensation on the equipment and the hardening and thus leakage of elastomeric seals.
One example of the difficulties that can arise in supplying a non-permanent gas from a cylinder is now given. It is desirable to store nitrous oxide in relatively small cylinders because these are easier to handle than in larger ones. A cylinder having a 0.5 l contains 240 g of nitrous oxide in the liquid phase, nominally enough for 20 minutes of delivery to a typical adult undergoing analgesia or anaesthesia with nitrous oxide. The latent heat of vaporisation of nitrous oxide at 00 C is about 250 kJ/kg. The total heat of evaporation is approximately 60 kJ in this example. Over 20 minutes of analgesia, this would typically average 50 W of cooling. The thermal mass of a typical aluminium cylinder and valve in a cylinder having a 0.5 l water capacity is 450 J/K. Without any heat transfer to the outside, the temperature drop of the cylinder during withdrawal of the nitrous oxide would be 60 kJ/450 J/K which is approximately 133K.
In practice, the temperature drop would be less than this, perhaps only 80K in total, as there will be some heat transfer from the surroundings of the cylinder, and some demand for heat will be lost with the gaseous nitrous oxide as it is delivered to a patient. However, a temperature drop of 80K is unacceptable because the external surface of the cylinder would be unsafe to touch owing to its low temperature and as the freezing point of nitrous oxide is approximately −80 C, there would be a risk of the nitrous oxide actually freezing within the cylinder. Further, many elastomeric sealing materials harden and leak if subject to a temperature less than −300 C.
Simple solutions to the problem are not effective. Thermally insulating the exterior of the cylinder would protect the user from cold burns, but exacerbate the temperature drop within the cylinder. Taking the nitrous oxide from the cylinder as a liquid, for example, via a dip-tube would have the advantage of substantially lessening the cooling effect on the cylinder, but displacing it to another place in the gas deliver equipment, for example, in a regulator, where there may be substantially less heat capacity with the result that even larger temperature drops may occur, causing freezing and condensation. Increasing the weight of the cylinder would reduce the temperature drop, but this may add unacceptably to the overall weight of the equipment.
Canadian Patent No. 1 061 578 relates to the supply of carbon dioxide from small pressurised gas capsules of the kind having a sealed mouth instead of a valve. According to Canadian Patent No. 1 061 578, a container is provided to hold a buffer substance, the container being in a heat conductive relationship with the capsule. The buffer substance is one that undergoes a change in its physical, chemical, crystallographic or other state at a temperature between ambient temperature and the final operating temperature of the capsule, the change of state causing a release of heat to the boiling liquefied gas. The heat is typically derived from latent heat of fusion of the buffer substance. The container of buffer substance may be located within the gas storage vessel or outside of it. If located externally, the container may take a form of jacket surrounding the vessel. The jacket is not self-defined, that is it holds the buffer substance in direct physical contact with the exterior of the gas storage vessel. This arrangement has a number of disadvantages. Prolonged contact of the buffer substance with the gas vessel may cause corrosion or erosion of the surface of the latter. Further, over a period of time the buffer substance becomes depleted by evaporation.